GFCI devices are utilized to interrupt a circuit path, typically at an AC receptacle, in response to the detection of a ground fault condition at an AC load. The input to an AC receptacle is generally an AC source (also called an AC line), and has two sides—the “hot” side (also sometimes called the “line” side), and the “neutral” side. A ground fault typically occurs when a short-circuit is created between the hot side of an AC source and an earth ground. For example, a ground fault condition results when a person comes into contact with the hot side of the AC load and an earth ground at the same time, a situation which can result in serious injury.
Typically, a GFCI device detects this condition by using a sensing transformer or wire coil to detect an imbalance between the currents flowing in the hot and neutral conductors of the AC supply, as will occur when some of the current on the line side is being diverted to ground. When such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the AC load.
In other instances, a ground fault may arise from the neutral conductor being grounded. This type of ground fault is dangerous because, if a person comes into contact with the hot side of the AC load when a grounded-neutral fault exists, a portion of the current flowing through the person will find a return path through the neutral-ground fault, potentially reducing the current imbalance in the hot and neutral conductors below the detection threshold. In other words, despite the hot-to-ground fault that is causing the person to be electrocuted, a GFCI device with a grounded-neutral side might not interrupt the circuit as intended.
A neutral-to-ground fault also presents direct detection problems. Because the neutral conductor is already at or near ground potential, this type of ground fault condition (i.e., a grounded-neutral fault) is difficult to detect at the sensing transformer because the neutral-to-ground fault may not cause a sufficient leakage current, or a current imbalance. Some GFCI detectors remedy this issue by providing a second transformer—a grounded-neutral transformer—which, when a grounded-neutral fault occurs, becomes magnetically coupled to the sensing transformer through the neutral conductor. The grounded-neutral condition is detected by providing an oscillating signal to the grounded-neutral transformer, usually by connecting the grounded-neutral transformer to the AC power source through an appropriate circuit, or by using a separate oscillator circuit. Because the neutral conductor acts as a shorted or low-impedance one-turn winding between the two cores of the transformers, the oscillations appear as an imbalance at the sensing transformer, and the GFCI device trips the circuit accordingly.
Although GFCI devices are commonly integrated into wall-mounted AC receptacles, GFCI devices have been provided in various forms, including portable or line cord devices and central units which provide protection for the AC wiring throughout a structure. A typical receptacle configuration consists of a housing adapted to be received within a standard electrical box, with a pair of standard two- or three-prong AC outlets, a test pushbutton and a reset pushbutton accessible through the front of the housing. Also accessible on the outside of the housing are input and output terminals—typically, two pairs of screw terminals. In some receptacle configurations, the input pair of screw terminals (also called the AC line terminals) and the output pair of screw terminals (also called the AC load terminals) are located on the sides of the receptacle, with each pair of terminals having a hot terminal and a neutral terminal positioned across from each other on opposing sides. In other receptacle configurations, the input pair of screw terminals are located on the sides of the receptacle, while the output pair of screw terminals are located on the front of the receptacle between the AC outlets.
The input pair of screw terminals are generally connected to the electrical outlets at the front of the housing via the GFCI circuitry within the housing. The output pair of screw terminals are connected directly in parallel with the contacts of the AC outlets by, for example, an electrical backplane. This output terminals provide the installer with the option of connecting a standard, non-GFCI AC receptacle in parallel with the GFCI receptacle, in order to provide ground fault protection for the standard receptacle without the need to provide a separate GFCI circuit. The standard receptacle may be located remotely from the GFCI receptacle, but will ordinarily be close enough (e.g., in the same room) so that convenient resetting is possible when a ground fault condition occurs.
Unfortunately, there is a problem with the GFCI receptacles described above, because an installer may erroneously connect the incoming AC source to the output terminals of the receptacle rather than to the input terminals. Because of the nature of the internal wiring of the GFCI receptacle, this mis-wiring condition is not easily detected. AC power will still be present at the receptacle outlets and at the output terminals, making it appear that the receptacle is operating normally. If the test pushbutton is depressed, the GFCI circuit within the receptacle will open a set of switches (e.g., a circuit breaker) and the reset button will pop out, again making it appear that the GFCI receptacle is operating normally and providing the desired ground fault protection.
In reality, however, no such protection is being provided because the AC source has been wired directly to the output terminals without passing through the internal circuit breaker of the GFCI device. The GFCI appears to be working properly because of the way the test button is wired—it electrically connects the hot and neutral sides of the electrical connection through the GFCI circuit, causing an unbalanced current flow between the two sides sufficient to trip the GFCI circuit, despite the fact that the AC source current is not normally flowing through the circuit. Thus, this “test” does not detect mis-wiring of the GFCI device.
In the past, GFCI manufacturers have addressed the GFCI mis-wiring problem in different ways. For example, various types of indicators have been utilized to alert an installer that a GFCI receptacle is mis-wired. In one design, GFCI circuits utilize a visual indicator (such as a light-emitting diode) that indicates that the GFCI circuit is operating normally. If the receptacle has been wired properly, the LED is extinguished when a ground fault condition occurs, or when the test button is depressed to simulate a ground fault condition. However, in the event that the receptacle has been mis-wired by connecting the AC source to the load terminals rather than to the input terminals, the LED is not extinguished when a ground fault condition occurs or when the test button is depressed. In other GFCI designs, a visual indicator illuminates directly when a mis-wiring occurs, provided that the switches of the GFCI circuit are in an open position and inversely coupled to an extra, third switch that is closed, thereby powering the visual indicator, which is electrically connected to the load side of the circuit.
In addition to visual indicators, audible alarms have been utilized in GFCI devices to provide a warning signal to an installer when a mis-wiring has occurred. The alarm does not stop sounding the warning until the GFCI circuit has been wired correctly. (Some GFCI circuits also include a timing circuit that triggers the alarm at pre-defined periodic intervals, such as every month, that does not turn off until a user has tested the GFCI circuit by pressing the test push-button). In some instances, the mis-wiring alarm has been combined with a tripping mechanism for the GFCI circuit.
Another existing technique to address the mis-wiring problem includes the use of a removable barrier interposed between one of the contacts in the circuit breaker to prevent the reset push-button from popping out when the test button is depressed, if the AC source is mis-wired to the input terminals. By utilizing an extra set of contacts (i.e., three contacts) in the circuit breaker, the removable barrier can block current from flowing from the mis-wired output terminals back through the GFCI circuit when the test button is pressed, but current can still flow through the GFCI when the test button is depressed and the AC source is wired correctly. (However, if the AC source is correctly wired to the input terminals, current cannot flow to the output terminals until the removable barrier (e.g., a mylar strip) is removed. Once removed, the barrier cannot be replaced because of the dangers of having an installer interfere with the GFCI circuit).
For further information on existing mis-wiring detection techniques, U.S. Pat. No. 5,600,524, U.S. Pat. No. 5,706,155, and U.S. Pat. No. 5,638,243 may be helpful.
While these techniques provide some means to determine when a mis-wiring condition has occurred, they tend to add substantial manufacturing complexity, have limited use, and/or require detailed, advance knowledge about the particular operation of the GFCI circuit. Thus, there is a need for a multiple-use GFCI mis-wiring detector that provides a simple notification of a mis-wiring, and that is relatively inexpensive to manufacture.